cfg80211: util: fix bit count off by one

The bits of Rx MCS Map in VHT capability were enumerated
with index transform - index i -> (i + 1) bit => nss i. BUG!
while it should be -   index i -> (i + 1) bit => (i + 1) nss.

The bug was exposed in commit a53b2a0b ("iwlwifi: mvm: implement VHT
extended NSS support in rs.c"), where iwlwifi started using the
function.
Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Fixes: b0aa75f0 ("ieee80211: add new VHT capability fields/parsing")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/util.c

@@ -1998,7 +1998,7 @@ int ieee80211_get_vht_max_nss(struct ieee80211_vht_cap *cap,
 			continue;
 
 		if (supp >= mcs_encoding) {
-			max_vht_nss = i;
+			max_vht_nss = i + 1;
 			break;
 		}
 	}